Tracer Studies of Acid-Catalyzed Reactions. I. The Isomerization of Cyclopropane over Silica-Alumina John G. Larson, H. R. Gerberich, and W. Keith Hall
Contribution f r o m the Mellon Institute, Pittsburgh, Pennsylvania. Received December 3, 1964 When small slugs of cyclopropane were passed over exhaustively deuterated silica-alumina catalysts, deuterium atoms were introduced into only about 10% of the product propylene molecules; the unisomerized cjsclopropane was exchanged to about the same extent. Multiple deuteration was not appreciable except when DLO was added back or when the ratio of catalyst to cyclopropane was made high. When exposed to the cataljst under identical conditions, propylene became deuterated to an extent comparable with the propylene produced bj? isomerization. Evidently, most of the deuterium which appears in these hydrocarbons results f i o m an exchange process separate f r o m the isomerization mechanism. When equimolar mixtures of cyclopropane and cjsclopropane-d6 were passed over the catalyst, both the propjlene and the residual cyclopropane became extensively deuterated. It was shown that multiple deuteration of the cj'clopropane occurred bj? a stepwise process and that the initial (low conversion) distribution of the product propylene contained approximately equimolar amounts of CaH6, CaHSD, CODSH, and C 3D6. The isomerization and isotope mixing reactions had the same activation energy. There were no firstorder. isotope effects. The results m a y be interpreted alternativelj. in terms of a carbonium ion mechanism, making repeated use of a small number ( < 1012/cm.2) of catalj-st protons or cocatalytic hydroxyl groups, or as a bimolecular hydride transfer mechanism involving cyclopropyl (C3H5+)carbonium ions. Alumina was a somewhat poorer catalyst f o r isomerization, but a better catalyst f o r exchange; silica gel was not a catalyst and did not undergo exchange with cyclopropane up to 500".
reported an isotope effect of 1.98 for the coisomerization of cyclopropane and cyclopropane-d8 at 482"; no isotopic mixing occurred. The heterogeneous, acid-catalyzed isomerization has also been studied.'O-12 The reaction goes at comparable rates at considerably lower temperatures (150 vs. 600") and with a much lower apparent activation energy (ca. 17 kcal./mole over silica-alumina); the rate is first order in cyclopropane pressure and appears to parallel the "water content" of the catalyst. * * These facts are consistent with the suggestion of Roberts'O that the cyclopropane ring is opened when a catalyst proton is attached to a ring carbon forming a n-propylcarbonium ion; since the added proton forms a methyl group, the isomerization is completed by loss of another proton to the catalyst surface. The purpose of the present work was to test this hypothesis. The Roberts mechanismlo predicts that when a completely deuterated catalyst is contacted with cyclopropane, the initial product propylene should contain nearly one deuterium atom per molecule; deuteration of unreacted cyclopropane is not anticipated. A primary isotope effect would be expected for the isomerization reaction, i.e., k H / k U 2 . Two limiting cases can be visualized when small slugs of cyclopropane are passed over an exhaustively deuterated catalyst using the microcatalytic technique, depending upon the ratio of cyclopropane molecules, ng, per slug to the available Df, n,. If n, >> n g , a plot of the fraction of C3H,D in the product propylene vs. slug number should have a gentle slope; if n,
